The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle

Science ◽  
2019 ◽  
Vol 363 (6431) ◽  
pp. 1080-1084 ◽  
Author(s):  
Adam P. Hasenfratz ◽  
Samuel L. Jaccard ◽  
Alfredo Martínez-García ◽  
Daniel M. Sigman ◽  
David A. Hodell ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Residence Time ◽  
Surface Waters ◽  
High Amplitude ◽  
Ice Age ◽  
Ice Ages ◽  
Glacial Cycle ◽  
Surface Circulation ◽  
Carbon Dioxide Release ◽  
The Antarctic

From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5 million years, we found that a reduction in deep water supply and a concomitant freshening of the surface ocean coincided with the emergence of the high-amplitude 100,000-year glacial cycle. We propose that this slowing of deep-to-surface circulation (i.e., a longer residence time for Antarctic surface waters) prolonged ice ages by allowing the Antarctic halocline to strengthen, which increased the resistance of the Antarctic upper water column to orbitally paced drivers of carbon dioxide release.

Climate Response to External Sources of Freshwater: North Atlantic versus the Southern Ocean

2007 ◽  
Vol 20 (3) ◽  
pp. 436-448 ◽  
Author(s):  
Ronald J. Stouffer ◽  
Dan Seidov ◽  
Bernd J. Haupt
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
North Atlantic ◽  
Surface Waters ◽  
Real World ◽  
The Other ◽  
Sea Surface ◽  
The North ◽  
The North Atlantic ◽  
The Antarctic

Abstract The response of an atmosphere–ocean general circulation model (AOGCM) to perturbations of freshwater fluxes across the sea surface in the North Atlantic and Southern Ocean is investigated. The purpose of this study is to investigate aspects of the so-called bipolar seesaw where one hemisphere warms and the other cools and vice versa due to changes in the ocean meridional overturning. The experimental design is idealized where 1 Sv (1 Sv ≡ 106 m3 s−1) of freshwater is added to the ocean surface for 100 model years and then removed. In one case, the freshwater perturbation is located in the Atlantic Ocean from 50° to 70°N. In the second case, it is located south of 60°S in the Southern Ocean. In the case where the North Atlantic surface waters are freshened, the Atlantic thermohaline circulation (THC) and associated northward oceanic heat transport weaken. In the Antarctic surface freshening case, the Atlantic THC is mainly unchanged with a slight weakening toward the end of the integration. This weakening is associated with the spreading of the fresh sea surface anomaly from the Southern Ocean into the rest of the World Ocean. There are two mechanisms that may be responsible for such weakening of the Atlantic THC. First is that the sea surface salinity (SSS) contrast between the North Atlantic and North Pacific is reduced. And, second, when freshwater from the Southern Ocean reaches the high latitudes of the North Atlantic Ocean, it hinders the sinking of the surface waters, leading to the weakening of the THC. The spreading of the fresh SSS anomaly from the Southern Ocean into the surface waters worldwide was not seen in earlier experiments. Given the geography and climatology of the Southern Hemisphere where the climatological surface winds push the surface waters northward away from the Antarctic continent, it seems likely that the spreading of the fresh surface water anomaly could occur in the real world. A remarkable symmetry between the two freshwater perturbation experiments in the surface air temperature (SAT) response can be seen. In both cases, the hemisphere with the freshwater perturbation cools, while the opposite hemisphere warms slightly. In the zonally averaged SAT figures, both the magnitude and the pattern of the anomalies look similar between the two cases. The oceanic response, on the other hand, is very different for the two freshwater cases, as noted above for the spreading of the SSS anomaly and the associated THC response. If the differences between the atmospheric and oceanic responses apply to the real world, then the interpretation of paleodata may need to be revisited. To arrive at a correct interpretation, it matters whether or not the evidence is mainly of atmospheric or oceanic origin. Also, given the sensitivity of the results to the exact details of the freshwater perturbation locations, especially in the Southern Hemisphere, a more realistic scenario must be constructed to explore these questions.

scholarly journals Weddell Sea Export Pathways from Surface Drifters

2015 ◽  
Vol 45 (4) ◽  
pp. 1068-1085 ◽  
Author(s):  
Madeleine K. Youngs ◽  
Andrew F. Thompson ◽  
M. Mar Flexas ◽  
Karen J. Heywood
Keyword(s):  
Continental Shelf ◽  
Wind Stress ◽  
Surface Waters ◽  
Weddell Sea ◽  
Surface Velocity ◽  
Wind Stress Curl ◽  
Weddell Gyre ◽  
Surface Circulation ◽  
Surface Drifters ◽  
The Antarctic

AbstractThe complex export pathways that connect the surface waters of the Weddell Sea with the Antarctic Circumpolar Current influence water mass modification, nutrient fluxes, and ecosystem dynamics. To study this exchange, 40 surface drifters, equipped with temperature sensors, were released into the northwestern Weddell Sea’s continental shelf and slope frontal system in late January 2012. Comparison of the drifter trajectories with a similar deployment in early February 2007 provides insight into the interannual variability of the surface circulation in this region. Observed differences in the 2007 and 2012 drifter trajectories are related to a variable surface circulation responding to changes in wind stress curl over the Weddell Gyre. Differences between northwestern Weddell Sea properties in 2007 and 2012 include 1) an enhanced cyclonic wind stress forcing over the Weddell Gyre in 2012; 2) an acceleration of the Antarctic Slope Current (ASC) and an offshore shift of the primary drifter export pathway in 2012; and 3) a strengthening of the Coastal Current (CC) over the continental shelf in 2007. The relationship between wind stress forcing and surface circulation is reproduced over a longer time period in virtual drifter deployments advected by a remotely sensed surface velocity product. The mean offshore position and speed of the drifter trajectories are correlated with the wind stress curl over the Weddell Gyre, although with different temporal lags. The drifter observations are consistent with recent modeling studies suggesting that Weddell Sea boundary current variability can significantly impact the rate and source of exported surface waters to the Scotia Sea, a process that determines regional chlorophyll distributions.

scholarly journals The Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific

2020 ◽  
pp. 106732
Author(s):  
Daniel M. Sigman ◽  
François Fripiat ◽  
Anja S. Studer ◽  
Preston C. Kemeny ◽  
Alfredo Martínez-García ◽  
...  
Keyword(s):  
Southern Ocean ◽  
North Pacific ◽  
Ice Ages ◽  
The North ◽  
The Antarctic ◽  
The North Pacific

scholarly journals New insights on the role of organic speciation in the biogeochemical cycle of dissolved cobalt in the southeastern Atlantic and the Southern Ocean

2012 ◽  
Vol 9 (7) ◽  
pp. 2719-2736 ◽  
Author(s):  
J. Bown ◽  
M. Boye ◽  
D. M. Nelson
Keyword(s):  
Southern Ocean ◽  
Surface Waters ◽  
Organic Ligands ◽  
Combined Effects ◽  
Deep Waters ◽  
Organic Complexes ◽  
Organic Speciation ◽  
Greenwich Meridian ◽  
The Antarctic ◽  
Southeastern Atlantic

Abstract. The organic speciation of dissolved cobalt (DCo) was investigated in the subtropical region of the southeastern Atlantic, and in the Southern Ocean in the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre, between 34°25´ S and 57°33´ S along the Greenwich Meridian during the austral summer of 2008. The organic speciation of dissolved cobalt was determined by competing ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using nioxime as a competing ligand. The concentrations of the organic ligands (L) ranged between 26 and 73 pM, and the conditional stability constants (log K'CoL) of the organic complexes of Co between 17.9 and 20.1. Most dissolved cobalt was organically complexed in the water-column (60 to >99.9%). There were clear vertical and meridional patterns in the distribution of L and the organic speciation of DCo along the section. These patterns suggest a biological source of the organic ligands in the surface waters of the subtropical domain and northern subantarctic region, potentially driven by the cyanobacteria, and a removal of the organic Co by direct or indirect biological uptake. The highest L:DCo ratio (5.81 ± 1.07 pM pM−1) observed in these surface waters reflected the combined effects of ligand production and DCo consumption. As a result of these combined effects, the calculated concentrations of inorganic Co ([Co']) were very low in the subtropical and subantarctic surface waters, generally between 10−19 and 10−17 M. In intermediate and deep waters, the South African margins can be a source of organic ligands, as it was suggested to be for DCo (Bown et al., 2011), although a significant portion of DCo (up to 15%) can be stabilized and transported as inorganic species in those DCo-enriched water-masses. Contrastingly, the distribution of L does not suggest an intense biological production of L around the Antarctic Polar Front where a diatom bloom had recently occurred. Here [Co'] can be several orders of magnitude higher than those reported in the subtropical domain, suggesting that cobalt limitation was unlikely in the ACC domain. The almost invariant L:DCo ratio of ~1 recorded in these surface waters also reflected the conservative behaviours of both L and DCo. In deeper waters higher ligand concentrations were observed in waters previously identified as DCo sources (Bown et al., 2011). At those depths the eastward increase of DCo from the Drake Passage to the Greenwich Meridian could be associated with a large scale transport and remineralisation of DCo as organic complexes; here, the fraction stabilized as inorganic Co was also significant (up to 25%) in the low oxygenated Upper Circumpolar Deep Waters. Organic speciation may thus be a central factor in the biogeochemical cycle of DCo in those areas, playing a major role in the bioavailability and the geochemistry of Co.

scholarly journals New insights on the role of organic speciation in the biogeochemical cycle of dissolved cobalt in the southeastern Atlantic and the Southern Ocean

2012 ◽  
Vol 9 (3) ◽  
pp. 3381-3422 ◽  
Author(s):  
J. Bown ◽  
M. Boye ◽  
D. M. Nelson
Keyword(s):  
Southern Ocean ◽  
Surface Waters ◽  
Organic Ligands ◽  
Biogeochemical Cycle ◽  
Combined Effects ◽  
Deep Waters ◽  
Organic Complexes ◽  
Organic Speciation ◽  
The Antarctic ◽  
Southeastern Atlantic

Abstract. The organic speciation of dissolved cobalt was investigated in the subtropical region of the southeastern Atlantic, and in the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre in the Southern Ocean between 33°58′S and 57°33′S along the Greenwich Meridian during the austral summer of 2008. The organic speciation of cobalt was determined by Competing Ligand Exchange Adsorptive Cathodic Stripping Voltammetry (CLE-AdCSV) using nioxime as a competing ligand. The conditional stability constants (log K'CoL) of the organic complexes of Co ranged between 17.9 and 20.1, and the concentrations of the organic ligands (L) between 26 and 73 pM. Most dissolved cobalt (DCo) was organically complexed in the water-column (60 to ≥99.9 %). There were clear vertical and meridional patterns in the distribution of L and the organic speciation of DCo along the section. These patterns suggested a biological source of the organic ligands in the surface waters of the subtropical domain and northern subantarctic region, potentially driven by the cyanobacteria, and a removal of the organic Co by direct or indirect biological uptake. The highest L:DCo ratio (e.g. 5.81 ± 1.07 pM pM–1) observed in these surface waters reflected the combined effects of ligand production and consumption of DCo. As a result of these combined effects, the calculated concentrations of free, unbound Co ([Co′]) in subtropical and subantarctic surface waters were very low, generally between 10–19 and 10–17 M. In intermediate and deep waters, the South African margins can be a source of organic ligands, as it was suggested to be for DCo (Bown et al., 2011), although a significant portion of DCo (up to 15 %) can be stabilized and transported as inorganic species in those DCo-enriched water-masses. Contrastingly, the distribution of L did not suggest an intense biological production of L around the Antarctic Polar Front where a diatom bloom had recently occurred. Here [Co′] can be several orders of magnitude higher than those reported in the subtropical domain, suggesting that cobalt limitation was unlikely in the ACC domain. The almost invariant L:DCo ratio of ~1 recorded in these surface waters also reflected the conservative behaviours of both the organic ligands and DCo. In deeper waters relatively higher ligand concentrations were observed in waters previously identified as DCo sources (Bown et al., 2011). At those depths the eastward increase of DCo could be associated with a large scale transport and remineralisation of DCo as organic complexes; here, the fraction stabilized as inorganic Co was much lower but still significant (up to 25 %) in the low oxygenated Upper Circumpolar Deep Waters. The organic speciation may thus be a central factor in the biogeochemical cycle of DCo in those areas, playing a major role in the bioavailability and the geochemistry of Co.

Modeling North American Freshwater Runoff through the Last Glacial Cycle

1999 ◽  
Vol 52 (3) ◽  
pp. 300-315 ◽  
Author(s):  
Shawn J. Marshall ◽  
Garry K.C. Clarke
Keyword(s):  
North America ◽  
North Atlantic ◽  
Ice Sheets ◽  
Ice Sheet ◽  
High Amplitude ◽  
Ice Age ◽  
Glacial Cycle ◽  
Ice Dynamics ◽  
Last Glacial ◽  
The Last Glacial

The Northern Hemisphere ice sheets decayed rapidly during deglacial phases of the ice-age cycle, producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation. The continental record of ice-sheet history is more obscured during the growth and advance of the last great ice sheets, ca. 120,000–20,000 yr B.P., but ice cores tell of high-amplitude, millennial-scale climate fluctuations that prevailed throughout this period. These climatic excursions would have provoked significant fluctuation of ice-sheet margins and runoff variability whenever ice sheets extended to mid-latitudes, giving a complex pattern of freshwater delivery to the oceans. A model of continental surface hydrology is coupled with an ice-dynamics model simulating the last glacial cycle in North America. Meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice-sheet margin. The coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins. Results imply an intensified surface hydrological environment when ice sheets are present, despite a net decrease in precipitation during glacial periods. Diminished continental evaporation and high levels of meltwater production combine to give mid-latitude runoff values that are highly variable through the glacial cycle, but are two to three times in excess of modern river fluxes; drainage to the North Atlantic via the St. Lawrence, Hudson, and Mississippi River catchments averages 0.356 Sv for the period 60,000–10,000 yr B.P., compared to 0.122 Sv for the past 10,000 yr. High-amplitude meltwater pulses to the Gulf of Mexico, North Atlantic, and North Pacific occur throughout the glacial period, with ice-sheet geometry controlling intricate patterns of freshwater routing variability. Runoff from North America is staged in the final deglaciation, with a stepped sequence of pulses through the Mississippi, St. Lawrence, Arctic, and Hudson Strait drainages.

Ocean carbon storage and release over a glacial cycle

2020 ◽  
Author(s):  
James Rae ◽  
Alan Foreman ◽  
Jessica Crumpton-Banks ◽  
Andrea Burke ◽  
Christopher Charles ◽  
...  
Keyword(s):  
Climate Change ◽  
Southern Ocean ◽  
Carbon Storage ◽  
Ocean Circulation ◽  
Deep Ocean ◽  
Ocean Surface ◽  
Ice Age ◽  
Glacial Cycle ◽  
Carbon Chemistry ◽  
Ocean Carbon

<p>Perhaps the most important feedback to orbital climate change is CO<sub>2</sub> storage in the deep ocean.  By regulating atmospheric CO<sub>2</sub>, ocean carbon storage synchronizes glacial climate in both hemispheres, and drives the full magnitude of glacial-interglacial climate change.  However few data exist that directly track the deep ocean’s carbon chemistry over a glacial cycle.  Here, we present geochemical reconstructions of deep ocean circulation, redox, and carbon chemistry from sediment cores making up a detailed depth profile in the South Atlantic, alongside a record of Southern Ocean surface water CO<sub>2</sub>, spanning the last glacial cycle.  These data indicate that initial glacial CO<sub>2</sub> drawdown is associated with a major increase in surface ocean pH in the Antarctic Zone of the Southern Ocean, cooling at depth, enhanced deep ocean stratification, and carbon storage.  Deep ocean carbon storage and deep stratification are further enhanced when CO<sub>2</sub> falls at the onset of Marine Isotope Stage 4, and are also pronounced during the LGM, illustrating a link between orbital scale climate stages and deep ocean carbon.  However our data also illustrate non-linear feedbacks to orbital forcing during glacial terminations, which show abrupt decreases in pH in Southern Ocean surface and subsurface waters, as CO<sub>2</sub> is rapidly expelled from the deep ocean at the end of the last ice age.</p>

scholarly journals Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age

2017 ◽  
Vol 114 (13) ◽  
pp. 3352-3357 ◽  
Author(s):  
Xingchen Tony Wang ◽  
Daniel M. Sigman ◽  
Maria G. Prokopenko ◽  
Jess F. Adkins ◽  
Laura F. Robinson ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Deep Sea ◽  
Ice Age ◽  
Nitrate Transport ◽  
Surface Ocean ◽  
Unified View ◽  
Desmophyllum Dianthus ◽  
Last Ice Age ◽  
The Antarctic ◽  
The Last Glacial

The Southern Ocean regulates the ocean’s biological sequestration of CO2 and is widely suspected to underpin much of the ice age decline in atmospheric CO2 concentration, but the specific changes in the region are debated. Although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core-based measurements, the use of different proxies in different regions has precluded a unified view of Southern Ocean biogeochemical change. Here, we report measurements of the 15N/14N of fossil-bound organic matter in the stony deep-sea coral Desmophyllum dianthus, a tool for reconstructing surface ocean nutrient conditions. The central robust observation is of higher 15N/14N across the Southern Ocean during the Last Glacial Maximum (LGM), 18–25 thousand years ago. These data suggest a reduced summer surface nitrate concentration in both the Antarctic and Subantarctic Zones during the LGM, with little surface nitrate transport between them. After the ice age, the increase in Antarctic surface nitrate occurred through the deglaciation and continued in the Holocene. The rise in Subantarctic surface nitrate appears to have had both early deglacial and late deglacial/Holocene components, preliminarily attributed to the end of Subantarctic iron fertilization and increasing nitrate input from the surface Antarctic Zone, respectively.

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